1. Field of the Invention
The invention relates to new metal compounds of monoesters of phosphoric acid as well as processes for their synthesis.
2. Description of the Prior Art
Phosphoric acid is a dibasic acid and exists in tautomeric equilibrium. ##STR3##
The equilibrium lies predominantly on the side of the asymmetric form II, which explains also the dibasic character of the phosphoric acid.
Ester compounds can be derived from both forms, namely, the triester of phosphoric acid and the diester of phosphoric acid. To some extent, these esters are susceptible to different reactions, as is also shown hereinafter.
The synthesis of monoesters of phosphoric acid, by the partial esterification of phosphoric acid with low molecular weight alcohols, is known, although the yields are unsatisfactory, since equilibrium mixtures of the dialkyl ester and the monoalkyl ester are produced and their separation is exceedingly difficult.
Furthermore, the hydrolysis of esters of phosphoric acid dichloride with water is known. In this reaction, it is difficult to protect the monoester compounds of phosphoric acid against further hydrolysis to phosphoric acid.
The synthesis of low-molecular-weight alcohol monoesters of phosphoric acid in the form of their sodium or ammonium salts by saponifying the corresponding diesters of phosphoric acid with alkaline or ammoniacal materials is also known.
In addition, it is known that the monobenzyl esters of phosphoric acid, if necessary, in the form of their ammonium salts, can be synthesized by reacting the dibenzyl ester of phosphoric acid with a tertiary amine, whereby a quaternary ammonium compound of the monobenzyl ester of phosphoric acid is formed which corresponds to the following formula: ##STR4## R = benzyl group R' -- low-molecular-weight alkyl
After removal of excess tertiary amine, the quaternary ammonium compound is converted with hydrochloric acid into the free monoester compound of phosphoric acid which is extracted with chloroform. The ammonium salt of the monoester compound of phosphoric acid is precipitated by passing in ammonia at 0.degree. C.
It is furthermore known that dibenzyl esters of phosphoric acid can be partially debenzylated by partially debenzylating the diesters by reaction with sodium iodide or ammonium iodide in butanone or with lithium chloride in ethoxyethanol.
In addition, it is known that dialkyl esters of phosphoric acid, with low-molecular-weight alkyl groups, can be partially split by reaction with ammonium thiocyanate.
Another method for preparing such ester compounds utilizes the reaction of low-molecular-weight dialkyl esters of phosphoric acid in boiling toluene with the sodium salt of low-molecular-weight dialkyl esters of phosphoric acid according to the following equation: ##STR5## whereby, in addition to the sodium salt of monoalkyl esters of phosphoric acid with low-molecular-weight alkyl residues, dialkyl esters of alkane phosphoric acid are formed.
The synthesis of monoesters of phosphoric acid by the hydrolysis of metaphosphoric acid esters, which, in turn, are obtained by the reaction of esters of phosphoric acid dichloride with metal oxides in which the metal halides are split off is also known. This proceeds according to the following equation: ##STR6##
Also, low-molecular-weight monoalkyl esters of phosphoric acid can be synthesized by hydrolysing diesters of acyl phosphoric acid, such as, those obtained by reacting salicylic acid with phosphorus (III) chloride according to the following reaction equation: ##STR7## and subsequently isolating the product as the iron (III) or aniline salt by precipitation.
It is furthermore known that monoesters of phosphoric acid with low-molecular-weight alkyl residues hydrolyze rapidly in acid solutions and slowly in alkaline solutions.